In photography, exposure is the amount of light in an area. It is how much light gets on the film or on the digital sensor in your DSLR. This amount is determined by a concept of the exposure triangle. The three points are the shutter speed, aperture, and ISO. There is a balance between the length of time the film or sensor is exposed (shutter speed), the amount of light coming through the lens (aperture width), and the sensitivity of the film or sensor (ISO).
Shutter speed controls the length of time that the film or sensor is exposed to light. Shutter speeds are typically measured in fractions of a second, for example (1/15th of a second), 1/30, 1/60, 1/125. Slow shutter speeds allow more light into the camera sensor and are used for low-light and night photography, while fast shutter speeds help to freeze motion.
Aperture is the diameter of the opening inside the lens diaphragm. Typically this can be changed to allow or alter the amount of light coming through the lens. This is displayed in f-stops (f/1.4, f/4, f/32). This has a side effect of changing the depth of field.
ISO is the sensitivity of the film to light or the simulation of that idea on a digital camera sensor. The higher the ISO the more sensitive the film is to light (that is getting the correct exposure is faster with more sensitive film).
Here is an illustration to show the balance:
When you take a picture you must figure out how much or how little light is needed. This is called metering and is typically handled through your camera system (we will go into that at another time). If there is too much light in your image, you should lower the shutter speed or ISO or reduce the aperture. If there is too little light in your image, you would work the reverse. The exciting things come from balancing these three to get the artistic effect that you want. If you want to capture a fast moving subject, you need a fast shutter speed. Then the image may be too dark so you must balance that shutter speed by opening the aperture and/or increase the ISO. These are the basics of exposure in photography. Next time, I will discuss the pros and cons of using the different points of the exposure triangle and what artistic things you can do with them.